Coating compositions comprising tetrafluoroethylene polymers and aluminum hydroxychloride

ABSTRACT

Aluminium hydroxychloride is added as an adhesive to fluorocarbon polymer dispersions to form coating compositions useful e.g. for the manufacture of non-stick aluminium cookware.

This is a division of application Ser. No. 367,004, filed June 4, 1973,now U.S. Pat. No. 3,900,684.

This invention relates to fluorocarbon polymer compositions. Inparticular it relates to compositions including dispersions offluorocarbon polymers in liquid media together with an adhesive. Theinvention also relates to methods of making such compositions and tomethods of coating substrates using compositions in liquid media. Itfurther relates to articles made from or made by use of the fluorocarbonpolymer compositions.

According to this invention there is provided a fluorocarbon polymercomposition including a fluorocarbon polymer and an aluminiumhydroxychloride. Aluminum hydroxychloride may be represented by theformula

    Al.sub.n (OH).sub.p Cl.sub.q.r.H.sub.2 O

wherein 3n equals p + q, and r is greater than or equal to zero. Theymay be made by known methods.

Liquid compositions according to the invention have been found useful inpreparing fluorocarbon polymer-containing coatings on substrates. Inparticular, they may be used to produce such coatings on metalsespecially aluminum, for example polytetra-fluoroethylene-containingcoatings on metal cookware and a wide variety of other articles. It hasbeen found that the aluminium hydroxychloride acts as a convenient, andinexpensive, adhesive or binder for the fluorocarbon polymer.

Compounds known as aluminium hydroxychloride can usually only be givenan approximate formula. A preferred aluminium hydroxychloride for use inthe invention has an approximate formula

    Al.sub.2 (OH).sub.5 Cl.xH.sub.2 O

where x is approximately 2. The material of this formula is commerciallyavailable under the name Aluminium Chlorhydrate.

In preparing a composition according to the invention, an adhesivesolution may first be formed and subsequently blended with afluorocarbon polymer dispersion.

By fluorocarbon polymers there are included polytetrafluoroethylene(PTFE) and copolymers of tetrafluoroethylene with up to 15% by weight ofother monomers such as ethylene, vinyl chloride and hexafluoropropene.Preferred fluorocarbon polymers are PTFE and copolymers oftetrafluoroethylene with up to 5%, especially 0.05 to 2%, by weight ofother monomers such as ethylene, vinyl chloride and hexafluoropropane.Such preferred polymers are subsequently referred to as "TFE resins".

Preferred compositions according to the invention include an aqueousdispersion of a TFE resin especially PTFE in which aluminiumhydroxychloride has been dissolved or with which an aqueous solution ofaluminium hydroxychloride has been blended. Preferred quantities of TFEresin per 100 g of composition are from 10 to 50 g especially 20 to 40g. Preferred quantities of aluminium hydroxychloride are such that theweight of aluminium per 100 g of composition are 1 - 5 g. Preferably theweight of aluminium in the aluminium hydroxychloride per 100 g of TFEresin should be at least 3 g and a weight within the range 6 to 20 gespecially 6 to 9 g is advantageously used.

The compositions may include other components such as pigments andfillers such as graphite.

A method according to the invention of coating a substrate comprisesapplying a composition as above set forth in a liquid medium to thesubstrate to form a coating, and heating the coating to remove thevolatile components and form an adherent coating. More than one coat maybe applied in this way. This method may be used e.g. for bondingfluorocarbon polymer to metal surfaces e.g. aluminium surfaces.

When the composition includes an aqueous dispersion of a TFE resin asabove defined, the coating may be dried to remove substantially all thevolatile components of the coating and it may subsequently be sintered,preferably after the application to it of a top coat of an aqueous TFEresin dispersion not including an adhesive. By means of the sinteringprocess, an adherent coating of TFE resin may be obtained on thesubstrate.

The aqueous compositions of the invention containing dispersed TFEresins may be applied as coatings to a variety of substrates, includingglass, e.g. for non-stick oven ware and non-stick autoclave linings;ceramics; metal such as ferrous metals, for example cast iron, mildsteel, stainless steel and aluminium and its alloys; and compositesurfaces such as metals having a reinforcing coating, such as a sprayedceramic, metal or metal + metal oxide coatings produced e.g. by flamespraying.

The compositions are particularly suited to coating cooking utensils,for example frying pans, saucepans and bakeware or for oven linings. Inmaking cooking utensils, a blank may be coated and then formed, or analready formed utensil may be coated.

Compositions according to the invention may also be used to formadherent non-stick, low friction, coatings on many other articlesincluding industrial processing equipment including moulds, rollers,stirrers, mixers, chutes, hoppers and heat sealing jaws, domesticarticles such as iron sole plates, food mixers and ice separators andtools such as saw blades.

The substrates to which the coating compositions are applied should beclean and free from grease and unless they have a fritted surface arepreferably roughened, for example by abrading by grit blasting or byetching. Coatings may be applied by any of the conventional techniques,including spraying, dipping and brushing.

The coating method in which a topcoat not containing an adhesive isapplied over a primer coating containing an adhesive according to theinvention is suitable for any of these applications.

The preferred compositions of the invention are aqueous compositionsmade by the use of aqueous dispersion of PTFE prepared by an aqueouspolymerisation process for example as described in U.K. Pat.specification Nos. 689, 400 and 821, 353.

It should be ensured that a sufficient quantity of a suitable stabiliseris present in any aqueous dispersion of a fluorocarbon polymer used inthis invention. Stabilisers found suitable include non-ionic surfactantssuch as those sold by Rohm and Haas Company of Philadelphia U.S.A.,under the trademarks "Triton" DN65 and "Triton" X100. On the other handit has been found that an anionic surfactant such as sodium laurylsulphate does not provide adequate stabilisation in the presence ofaluminium hydroxychloride.

An advantage of coatings prepared by the method of the present inventionis that they do not use chromic acid which has previously been used forpreparing fluorocarbon polymer coatings. Chromic acid is known to betoxic and it is clearly desirable to be able to avoid its use incompositions which may be used to coat cookware.

When the composition contains the fluorocarbon polymer dispersed in aliquid medium, the dispersion may be in an aqueous or an organic mediumand a dispersion may be made by a polymerisation process in an aqueousor an organic medium or by dispersing a suitably finely divided solidform of the polymer in a liquid medium. In the case of PTFE for example,the polymer may be used in the form of a dispersion obtained from apolymerisation process in aqueous or organic media or a dispersion maybe made by comminuting a suitable solid form of PTFE and dispersing itin a liquid, the comminution being effected preferably while the PTFE issuspended in a liquid medium in the presence of a dispersing agent.Lubricant grades of PTFE such as those sold by Imperial ChemicalIndustries Limited under the Trade Marks L170 and L171 may be used.Sufficient dispersing agent of a suitable kind is used to make a stabledispersion which will remain stable after addition of aluminiumhydroxychloride.

The invention will be illustrated with reference to the followingexamples.

EXAMPLE 1

35.8 g. of a commercially available aqueous aluminium hydroxychloridesolution (made from an aluminium hydroxychloride (also known asaluminium chlorhydrate) of approximate formula Al₂ (OH)₅ Cl.2H₂ Ocontaining 11.6% w/w aluminium and 8.6% w/w chloride was mixed with110.9 g of water and added with stirring to 103.3 g of an aqueous PTFEdispersion containing 60.5% w/w PTFE stabilised with 2.5% by weightrelative to PTFE of a non-ionic surfactant sold by Rohm and Haas Companyof Philadelphia, U.S.A. as "Triton" DN65 and described by the suppliersas a modified ethoxylated straight chain alcohol. The final mixture thuscontained by weight, 25% PTFE and 6.65% aluminium relative to PTFE. TheAl:Cl atomic proportions were thus 1:0.56. The primer was sprayed on toan aluminium plaque (previously grit blasted with 60 mesh alumina togive a Permascope reading of 16 μm). The plaque was then dried and bakedat 150°C for 10 minutes. The Permascope reading was then 18 μm. Aconventional pigmented PTFE-containing top coat was then sprayed on,dried for 5 minutes at 80°C and then sintered at 400°C for 10 minutes.The Permascope reading was then 30 μm. The coating appearance was good.Adhesion was assessed by tests as follows:

1. By cutting through the coating and into the metal with a razor bladeto give a cross-hatched pattern of cuts providing approximately 3 mm × 3mm squares over an area of 625 mm². The adhesion of the coating is thenassessed by firmly pressing a length of 25 mm wide, pressure-sensitive,adhesive tape over the cross-hatched area and subsequently pulling thetape sharply backwards at an angle of 45° to the direction ofapplication. The application and stripping of tape over thecross-hatched area is repeated with fresh tape until there is evidencethat the coating is detaching from the metal or, if no failure isobserved, up to a maximum of 15 times.

2. The cross-hatching procedure of test 1 was repeated and the testplaque was then immersed in a vegetable based cooking oil at 200°C for 8hours. The adhesive tape test is carried out, after washing the plaquewith detergent and drying it, up to a maximum of 15 times.

The results by these tests were excellent i.e. the coating withstood 15consecutive applications of tape both before and after immersing thecoating plaque in hot cooking oil as above described. Some details ofthis and following examples and results of tests relevant to theexamples are summarised in the Table.

EXAMPLE 2

To 2.63 g of the commercial aluminium chlorohydrate solution describedin Example 1 was added 123.5 g water. This mixture after stirring wasadded to 26.0 g of a PTFE aqueous dispersion (containing 58.5% by weightof PTFE and stabilised with 2.58% by weight relative to PTFE of "Triton"DN65 (Trade Mark) surfactant. The mixture was stirred and filtered. Itwas then tested as a primer as in Example 1 with the one modificationthat in the second test the plaque was immersed in the oil at 200°C for3 hours instead of 8 hours.

EXAMPLE 3

This was similar to Example 2, but the ingredients were now 26.3 g ofthe aluminium chlorohydrate solution, 100 g water and 26.0 g of PTFEdispersion. After filtering the mixture was tested as a primer as inExample 2.

EXAMPLE 4

This was similar to Example 2, but the ingredients were now 10.1 g ofthe aluminium chlorohydrate solution, 14.4 g water and 100 g of PTFEdispersion. After filtering, the mixture was tested as a primer as inExample 2.

EXAMPLE 5

24.9 g of aluminium chlorohydrate powder (analysing to show 23.5% byweight aluminium and 17.5% by weight of chloride) was mixed with 21.1 gwater and the mixture added to 100 g of PTFE dispersion of the type usedin Examples 2 to 4. After filtering the mixture was tested as a primeras in Example 2.

EXAMPLE 6

To 140.5 g of an aqueous PTFE dispersion (58.6% w/w PTFE), stabilisedwith "Triton" DN65 (2.58% w/w relative to PTFE) was added 144.5 g ofwater, followed, with stirring, by 45 g of a commercial aluminiumchlorohydrate [Al₂ (OH)₅ Cl] solution containing 12.2% by weightaluminium and 8.13% by weight chloride, followed by 3.3 g of an aqueousblack pigment paste (consisting of 25% by weight carbon black in water,emulsified with ca 5% by weight on total paste of a polyethoxylated longchain alcohol surfactant). The mixture was then filtered. This mix issimilar to that of Example 1 except that pigment is now present. It wastested as in Example 2.

EXAMPLE 7

This was similar to Example 6 but the quantities were now 175 g of PTFEdispersion, 111 g water, 56 g of aluminium chlorohydrate solution and3.4 g of pigment paste. It was tested as in Example 2.

EXAMPLE 8

This was similar to Example 6 but the quantities were now 249 g of PTFEdispersion, 37 g water, 79 g of aluminium chlorohydrate solution and 3.7g of pigment paste.

EXAMPLE 9

To 19.6 g of aluminium chlorohydrate solution (as used in Examples 6 -8) was added 34.3 g water followed by 4.6 g of concentrated hydrochloricacid (SG 1.18, 36% w/w HCl). This mix was poured with stirring into 61.5g of PTFE dispersion of the type used in Examples 6 to 8. The mix wasfiltered. It will be seen that the chloride content relative toaluminium is now approximately twice that of earlier examples, i.e. thechlorohydrate is now expressible as Al₂ (OH)₄ Cl₂. The mix was tested asa primer as in Example 2.

EXAMPLE 10

To 19.6 g of aluminium chlorohydrate solution as used in examples 6 to 8was added, 40.4 g water. The mix was added to 60 g of a PTFE dispersion(containing 60% w/w PTFE) stabilised with 3% by weight relative to PTFEof a surfactant sold by Rohm and Haas Company under the trademark"Triton" X100 which is an octyl phenol polyethylene oxide condensate.The mix was filtered and tested as a primer as in Example 2.

EXAMPLE 11

To 136.3 g of an aqueous PTFE dispersion (58.7% w/w PTFE) with a numberaverage median particle size of 0.14μ) stabilised by "Triton" DN65(2.58% w/w relative to PTFE), were added 37.6 g water, followed withstirring by 26.1 g of a commercial aluminium chlorohydrate [Al₂ (OH)₅Cl] solution containing 12.24% w/w aluminium and 8.30% w/w chloride.Test results are given in the accompanying table.

EXAMPLE 12

This mix was similar to Example 11 but now the weights were 102.2 g ofaqueous PTFE dispersion, 29.2 g water and 68.6 g aluminium chlorohydratesolution. See table for results.

EXAMPLE 13

This mix was similar to Example 11 but now weights were 102.2 g ofaqueous PTFE dispersion and 98.0 g of aluminium chlorohydrate solution.See table for results.

                                      Table                                       __________________________________________________________________________    Example                                                                            % PTFE                                                                             % Al in                                                                            % (Al )                                                                             Permascope readings Adhesive tape test                        in final                                                                           final                                                                               (PTFE)                                                                             (micron)            (No. of applications                 primer    primer     Substrate                                                                           With baked                                                                           After final                                                                          needed to cause grid                 (by weight)                primer coat                                                                          sintering                                                                            failure)                                                        present                                                                              of primer                                                                            Before hot                                                                          After hot                                                        and top coat                                                                         oil test                                                                            oil test                       __________________________________________________________________________    1    25.0 1.66 6.65  16    18     30     >15   >15                            2    10.0 0.20 2.00  15    17     30     8     6                              3    10.0 2.00 20.00 15    17     31     >15   >15                            4    47.0 0.94 2.00  16    18     31     4     3                              5    40.0 4.00 10.00 15    17     29     15    14                             6    24.8 1.65 6.65  15    16     28     15    14                             7    29.7 1.98 6.65  14    16     25     >15   >15                            8    39.6 2.63 6.65  13    14     25     >15   15                             9    30.0 2.00 6.65  15    16     28     15    14                             10   30.0 2.00 6.65  12    14     24     12    8                              11   40.0 1.60 4.00  17    18     30     8     5                              12   30.0 4.20 14.00 16    17     28     12    9                              13   30.0 6.00 20.00 15    17     30     +12   +6                             __________________________________________________________________________

We claim:
 1. A liquid composition comprising polytetrafluoroethylenepolymer or copolymer and an aluminium hydroxychloride wherein the weightof aluminum provided by the aluminum hydroxychloride is at least 3 g per100 g of polymer or copolymer.
 2. A composition according to claim 1where the aluminium hydroxychloride has the formula

    Al.sub.2 (OH).sub.5 Cl.xH.sub.2 O

where x is approximately
 2. 3. A composition according to claim 1wherein the polymer is polytetrafluoroethylene or a copolymer oftetrafluoroethylene with up to 5% by weight of another monomer.
 4. Acomposition according to claim 1 wherein the polymer and the aluminiumhydroxychloride are dispersed in an aqueous medium.
 5. A compositionaccording to claim 1 wherein the polymer and the aluminiumhydroxychloride are dispersed in an organic medium.
 6. A compositionaccording to claim 3 wherein the polymer and the aluminumhydroxychloride are dispersed in an aqueous medium, the weight of thepolymer being from 10 to 50 g per 100 g of the aqueous composition.
 7. Acomposition according to claim 1 wherein the weight of aluminum providedby the aluminum hydroxychloride is from 1 to 5 grams per 100 grams ofcomposition.
 8. A composition according to claim 1, wherein thecopolymer is a copolymer of tetrafluoroethylene with up to 15% by weightof other ethylenically unsaturated monomers.
 9. A composition accordingto claim 8 wherein the other unsaturated monomers are selected from thegroup consisting of ethylene, vinyl chloride and hexafluoropropane.